This invention relates generally to a method and apparatus for reducing uranium oxide by reaction with magnesium in a molten-salt medium of high density, thereby generating and isolating uranium as bulk metal. In this disclosure, unless otherwise specified, the term "uranium" is defined to include substantially metallic alloys comprising at least 50 percent by weight of the element uranium. Likewise, "uranium oxide" is defined to include substantially oxide mixtures of solutions comprising at least 50 percent by weight of oxides of the element uranium. Thus, for example, "uranium" refers to (among other materials) a militarily useful alloy produced from elemental uranium plus 7.5% niobium. If this alloy is burned, the product is uranium oxide, even though niobium oxide is present. Later reduction of this uranium oxide would return it substantially to an alloy containing 7.5% niobium, again to be included under the term "uranium."
Because much uranium scrap can be pyrophoric or flammable, such scrap often is deliberately burned before storage or burial as radioactive waste. There now is no simple way to return such oxidized scrap to useful metal.
Magnesium is used commercially as a reactant for converting uranium fluoride to uranium in bomb reductions. Such reductions involve preheating the mixtures of magnesium-uranium fluoride reactants prior to initiation of the reduction reaction. If thus preheated, the hot mixtures will fuse to molten products when supplied the additional heat of the reduction reaction. Even though such fusions produce high pressures of magnesium vapor, with corollary need for expensive high-pressure vessels, still the fusions are essential for separation of uranium (the product) for magnesium fluoride (the byproduct).
Likewise, following much European practice, in commercial reductions of uranium oxide by calcium in bombs, excess calcium is added over that for reaction stoichiometry so that remaining calcium will dissolve the calciumoxide byproduct and form a molten phase which will allow the molten uranium to separate. Again, pressures are high, bombs are expensive, and the use and handling of calcium are both more expensive and more difficult than with similar processes using magnesium (which, unlike calcium, is stable in air). Therefore, such calcium reductions are seldom used commercially in U.S. practice.
Because most current recycling of uranium involves the formation of uranium oxide, and because conversion of uranium oxide to uranium fluoride is expensive, it would be economically advantageous if means were available to reduce uranium oxide directly with magnesium. It would be of further advantage to be able to carry out the reduction at atmospheric pressure. Therefore, there is economic value and need for the present invention.
Several physical and chemical properties may be employed to make possible economical magnesium reductions of uranium oxide as in the present invention:
The very different densities of molten uranium (about 17.9 grams per cubic centimeter at 1150.degree. C.) and solid magnesium oxide (about 3.4 grams per cubic centimeter at 1150.degree. C.) could be used for the separation of product from byproduct if a substantially unreactive molten-salt phase of intermediate density could be selected. Very few compounds, however, offer a satisfactory combination of chemical stability against the reactants and products, suitable melting temperatures, and densities greater than 3.4 grams per cubic centimeter (to float solid magnesium oxide). Here one is limited to the chemically substantially identical lanthanides (lanthanum plus rare earths) plus strontium and barium for the cations with bromine and iodine for the anions. These bromides or iodides could be used alone or together as the required molten-salt solutions to float magnesium oxide and sink uranium.
Bromides are expensive, but iodides are more expensive, so bromides will usually be chosen as anions. Strontium and separated rare earths (except cerium) are expensive. Thus, most molten-salt solutions will contain BaBr.sub.2, LaBr.sub.3, or CeBr.sub.3 as the heavy salt. To save money and still keep a density of at least 3.4 grams per cubic centimeter, the heavy salts may be diluted with cheaper salts which are still fairly heavy, e.g., BaCl.sub.2, LaCl.sub.3, or BaCl.sub.3. In the following discussions, the term "molten-salt solution" is used, but it is understood that the term also covers substantially pure molten salts of density at least 3.4 grams per cubic centimeter.
The reactants have very different densities, as well. At 1150.degree. C. the densities are about 1.53 grams per cubic centimeter for molten magnesium and 10.7 grams per cubic centimeter for solid UO.sub.2. By making pellets of magnesium plus uranium oxide at rough reaction stoichiometry (e.g., 2Mg+UO.sub.2), it is possible to achieve pellets of densities which will sink in the said molten salt solutions. Such pellets, however, will not be stable when they become hot--rather, when they reach temperatures sufficient for reasonably rapid reaction to magnesium oxide and uranium products, they will tend to overheat locally, break apart, and disperse as small fragments which will settle (upward or downward) only slowly.
The different densities of molten uranium, molten salt, and solid oxides allows construction of traps which will pass molten uranium but retain molten salt and solid oxides. Thus, a tube placed vertically into a cup of molten uranium can be used to support a column of molten salt held up by the said molten uranium. For example, if the density of a molten salt (e.g., 3.5 grams per cubic centimeter) is one fifth the density of molten uranium (about 17.9 grams per cubic centimeter), a 5-cm depression of the molten uranium down the said tube will result from adding a 25-cm column of molten salt onto the molten uranium. The presence of uranium oxide (density about 10.7 grams per cubic centimeter) in the molten salt will cause a somewhat greater displacement of molten uranium from the said tube, but the said tube and cup of molten uranium can still be used to form a trap to pass molten uranium while retaining molten salt and solid oxide.
Solubilities of MgO, UO.sub.2, and Mg in the molten salt solutions of the present invention will vary with temperature and the solution components. Such solubilities may range to as much as several percent of a solution's composition. These solubilities are expected to be important to the present invention:
The solubility of MgO in the said molten-salt solutions is important because it will permit recrystallization of flocculent MgO precipitates into more compact crystals--as compared with flocculent precipitates, compact crystals are easier to drain (after being dipped out from a molten-salt solution) and are expected to be substantially freed of attached, unreacted UO.sub.2, an advantage for radioactivity removal. PA1 Likewise, dissolution of Mg in the said molten-salt solutions lowers the thermodynamic activity of the Mg, thereby allowing the Mg to be retained in solution at temperatures above the normal boiling point of the Mg (1090.degree. C.). Thus, even if the Mg does not immediately react with UO.sub.2, its chance of vaporization will be reduced by the said dissolution. In corollary, there will be reduced demand upon vapor-condensation and pumping devices needed to return vaporized Mg to the reaction zone. PA1 holding a molten-salt solution of density greater than 3.4 grams per cubic centimeter in a container means, PA1 adding uranium oxide to the said molten-salt solution, PA1 adding magnesium to the said molten-salt solution in the region of the said uranium oxide, PA1 reacting the said magnesium and the said uranium oxide, thereby forming magnesium oxide and uranium, PA1 floating the said magnesium oxide up from at least part of the zone in which the said magnesium and the said uranium oxide react together, and PA1 sinking the said uranium down from at least part of the said zone in which the said magnesium and the said uranium oxide react together. PA1 a container means which acts also as a conduit means, PA1 a trap means within the said container means, the said trap means being filled with molten uranium, and PA1 the said molten uranium supporting molten-salt solution of density greater than 3.4 grams per cubic centimeter.
Dissolution of some Mg and UO.sub.2 in the said molten-salt solution, spreads the regions in which the Mg-UO.sub.2 reactions can take place, thereby making the reaction faster and more complete that if the solubility did not exist.
The densities, solubilities, chemical stabilities, and concepts just discussed are coupled together in unique and unobvious ways to arrive at the method of the present invention. The need for the invention is demonstrated by the high cost of present practice in disposing of uranium oxide (particularly depleted uranium oxide from which most isotope U.sup.235 has been removed) waste and the high cost of purchases of new material to replace that which might have been recycled but, instead, was buried.